Cross Flow Twist Turbine

ABSTRACT

A cross flow turbine having one or more aerofoil blades rotatably mounted about a central axis and connected to said axis at or near each end of the one or more blades. The blades have a degree of torsional flexibility that make them twistable about the longitudinal blade axis to reduce the aerodynamic efficiency of the blades to control the rotational speed of the turbine. The twist of the blades can be actively controlled by means of a spring, other mechanical actuator or motor.

The invention relates to a turbine and in particular, but notexclusively to a turbine of the form where the operating fluid movessubstantially across the axis of rotation of the machine.

Wind turbines, and in particular horizontal axis wind turbines (HAWTs)are commonly used to harness the kinetic energy of wind to produceelectricity. HAWTs can be seen in many places across the country,mounted on large towers to catch the faster winds that blow at suchheights.

HAWTs have a rotor shaft and a generator mounted atop such towers, with(usually) three large turbine blades designed to convert a perpendicularairflow into rotational motion. The rotation of the rotor shaftgenerates electricity by means of the generator. Such turbines have hightip speed ratios, high efficiency and low torque ripple which increasesreliability.

As the towers on which the HAWTs are mounted generate turbulence, theturbine itself will most often be positioned upwind of the tower. For achange in wind direction from, say, NE to SW, this would require a 180°rotation of the turbine to resume. Some small turbines make use of awind vane to align the turbine with the wind. Other large turbines havewind direction sensors and motors to rotate the turbines automaticallyand optimise efficiency.

One drawback of realigning a turbine by rotation is that gyroscopicforces act on the blades as they rotate and the whole turbine turns.This causes twisting forces to be exerted on the turbine which canresult in fatigue and eventually damage to components of the turbine.

Savonius type wind turbines operate on a vertical axis, but aregenerally less efficient than lift producing turbines. Savonius typewind turbines are similar to anemometers, being that they have two orthree scoops arranged to catch the wind. The main benefit of suchturbines is that they require little maintenance, and are much cheaperthan similarly sized HAWTs. Additionally, there is no need to direct theturbine as they can operate with any cross flowing wind. However,Savonius turbines are inefficient as there is always a surface which issubject to some amount of drag. Hence Savonius turbines are known asdrag type systems.

Darrieus wind turbines (also known as “eggbeater” turbines) are anotherexample of vertical axis turbines. One of the benefits of vertical axisturbines is that the generator (which may be bulky and/or heavy) can belocated at the base of the turbine or on the ground. As with Savoniustype wind turbines, there is no requirement to point Darrieus windturbines into the wind. This is particularly advantageous for situationswhere the turbine is located in built up areas where nearby buildingscause increased wind turbulence.

Other advantages over HAWTs are that the blades have no tips or ends,and therefore there is no tip noise, turbulence or drag on blade ends.Additionally, the troposkien shape that the blades naturally assume meanthat there is no bending force on the rope or ropes therein, onlytensile forces distributed along the length of the rope(s).

Darrieus wind turbine devices have been in existence since 1931. In thattime very few significant advances have been made on the initial design.Commercially exploitable Darrieus turbines have been difficult toproduce for a number of reasons.

In general, they are low efficiency, which significantly limits theirpotential applications and their commercial viability. Also, largethrust loadings on the main bearings of such turbines means that bearingselection is critical.

The long blades of Darrieus turbines have many natural frequencies ofvibration which must be avoided during operation. Some turbines have twoor three rotational speeds that must be gone through quickly to reachoperating speed. Several modes may fall within the operational band andthus a control system should be used to avoid these modes.

When this type of machine is used in a variable speed fluid such asatmospheric wind flows there can be a problem controlling the rotationalspeed in high wind conditions. This is particularly problematic inefficient “lifting” type machines, such as HAWTs or Darrieus typeturbines, where destructive rotational speeds can be reached.

Therefore, another important consideration is that the rotational speedof the turbine must be limited in high wind conditions and variousattempts have been made to do so.

UK Patent Application 2,216,606 A in the name Jeronimidis et aldiscloses blades for use with turbines with a horizontal or verticalaxis of rotation. The blades exhibit an anisotropy which causes them tobend or stretch as the rotational speed increases. The bending and/orstretching affect the rotational speed of the blades as the angle ofattack is changed and the load on the blades is altered.

U.S. Pat. No. 4,500,257 discloses a braking system for a vertical axiswind turbine in which a block is slidably located on a blade. A solenoidreleases the block at a desired time and the block moves up the bladetowards its outermost point under centripetal force. The reducedaerodynamic efficiency reduces the rotational speed.

French Patent Application 2 583 823 shows a vertical axis wind turbinewhich has a drum or disk brake to implement a mechanical braking systemwhen the rotation of the turbine reaches a threshold speed.

Such drag devices and mechanical brakes have been proposed to limitrotational speeds in horizontal and vertical axis turbines. Drag devicescan be unreliable, and need to be maintained. Mechanical brakes arecumbersome and result in wear and tear on the system. Such methods oflimiting rotation may also impact on the smoothness of power output fromthe turbine.

An object of this invention is to provide aerodynamic limiting of theupper rotational speed of a turbine.

A further object of this invention is to provide aerodynamic limiting ofthe upper rotational speed of a turbine by twisting.

In accordance with a first aspect of the invention there is provided across flow turbine comprising:

one or more aerofoil blades rotatably mounted about a central axis andconnected to said axis at or near each end of the one or more blades

wherein said one or more blades are provided with a degree of torsionalflexibility such that they are twistable about a longitudinal blade axisto reduce the aerodynamic efficiency of the one or more blades tocontrol the rotational speed of the turbine.

Twisting the one or more aerofoil blades out of optimal lift conditionslimits the speed of rotation of the turbine by reducing forward drivingforces and increasing drag forces.

Preferably, the one or more blades are provided with a rotatableconnector to allow the blade to twist about the longitudinal blade axis.

Preferably, the rotatable connector couples the one or more aerofoilblades to the central axis at one end of the blade.

Optionally, the rotatable connector couples the one or more aerofoilblades to the central axis at both ends of the one or more blades.

Optionally, the rotatable connector is positioned a distance along thelongitudinal axis of the one or more aerofoil blades to couple twosections of the one or more aerofoil blades.

Optionally, rotation of the rotatable connector is driven by tension inthe one or more blades caused by centripetal force.

Preferably, the rotatable connector is provided with a rotationinhibiting means that prevents rotation below a predeterminedcentripetal force threshold.

Preferably, the rotation inhibiting means comprises a torsion springwound against a rotation stop which holds the one or more blades inplace.

Optionally, the rotation inhibiting means comprises one or more springsfixed at a helical angle to the central axis of rotation and to the oneor more blades at the other end.

Optionally, the rotation inhibiting means comprises two triangularsections of stiff material with flexible links therebetween, said linksforming a Z shape.

Preferably, the one or more aerofoil blades are configured to twist in apredetermined direction when a tension threshold is reached.

Preferably, the one or more aerofoil blades are configured to twist in afirst direction to feather turbine rotation.

Optionally, the one or more aerofoil blades are configured to twist in asecond direction to stall turbine rotation.

Optionally, rotation of the rotatable connector is driven by anactuator.

Preferably, the actuator operates at a predetermined threshold ofcentral axis rotational velocity.

Preferably, the actuator is powered.

Preferably, the actuator is manually controllable.

Optionally, the actuator is automatically controllable.

Preferably, the torsional flexibility of the one or more aerofoil bladesare set at a predetermined level.

Preferably, the torsional flexibility of the one or more blades can beengineered such that the degree of twist causes a proportional degree oftwist at the mid-point between the ends of the one or more blades.

Preferably, said level is set such that substantially 180° of twist atone end of the one or more blades causes substantially 90° of at themid-point between the ends.

This will effectively stop the driving force on the one or more blades.

Optionally, said level is set such that substantially 180° of twist atone end of the one or more blades causes 120° of twist at the mid-pointbetween the ends.

Optionally, said level is set such that substantially 180° of twist atone end of the one or more blades causes 60° of twist at the mid-pointbetween the ends.

Typically, the speed of rotation of the turbine will be controlled by alesser rotation at the one or more blade ends as any rotation willaffect the aerodynamic properties of the one or more blades and increasedrag.

Once the speed of rotation of the turbine had reduced to or below anacceptable operating level, the one or more blade ends will return totheir original position for optimum blade aerodynamics.

Preferably, the one or more aerofoil blades are capable of adopting atroposkien shape during rotation about the central axis.

Preferably, the one or more aerofoil blades comprise one or moreflexible ropes enclosed by an aerofoil shaped profile.

Preferably, the aerofoil shaped profile contains a packing material tomechanically fix the aerofoil shaped profile to the one or more ropes.

Preferably, the cross flow turbine further comprises connection meansprovided at an end of the one or more blades which is releasablyconnectable to the central axis such that when speed of rotation of theturbine about the central axis increases to or over a predeterminedthreshold level the one or more blades are released.

By releasing one end of the one or more aerofoil blades to fly out, theforward driving force of the one or more blades is reduced.

Excess tension in the one or more blades due to centripetal forcescaused by excess rotational speed can cause the one or more blade endsto be released.

This feature provides the present invention with a fail safe mechanismoperable in extreme weather conditions.

Preferably, the one or more aerofoil blades are flexible.

Preferably, the connection means is releasably connectable by means of aclamp.

Preferably, the cross flow turbine comprises a plurality of aerofoilblades each of which are releasably connectable and wherein release ofall blades occurs upon reaching said predetermined speed of rotationthreshold.

Preferably, said blades are released substantially simultaneously.

Preferably, a single mechanism is used to release all of the blades.

When the blade ends are released they swing out under centripetalforces. The resulting increase in diameter produces an increase inangular inertia which immediately slows the turbine. Further slowingthen occurs due to the adverse aerodynamic geometry of the blades whenheld at one end only.

In accordance with a second aspect of the invention there is provided across flow turbine comprising:

one or more aerofoil blades rotatably mounted about a central axis andconnected to the central axis at or near each end of the one or moreblades by connection means wherein

the connection means provided at one end of the one or more blades isreleasably connectable and is released when speed of rotation of theturbine about the central axis increases to or over a predeterminedthreshold level.

By releasing one end of one or more aerofoil blades to fly out, theforward driving force of the one or more blades is reduced.

Excess tension in the one or more blades due to centripetal forcescaused by excess rotational speed can cause the one or more blade endsto be released.

This feature provides the present invention with a fail safe mechanismoperable in extreme weather conditions.

Preferably, the one or more aerofoil blades are flexible.

Preferably, the connection means is releasably connectable by means of aclamp.

Preferably, the cross flow turbine comprises a plurality of aerofoilblades each of which are releasably connectable and wherein release ofall blades occurs upon reaching said predetermined speed of rotationthreshold.

Preferably, said blades are released substantially simultaneously.

Preferably, a single mechanism is used to release all of the blades.

When the blade ends are released they swing out under centripetalforces. The resulting increase in diameter produces an increase inangular inertia which immediately slows the turbine. Further slowingthen occurs due to the adverse aerodynamic geometry of the blades whenheld at one end only.

The invention will now be described by way of example only withreference to the accompanying drawings in which:

FIG. 1 shows a view of a twist type turbine perpendicular to the axis ofturbine rotation;

FIG. 2 shows a view along the axis of turbine rotation;

FIG. 3 (Detail A) shows a representation of a rotatable end fixing for ablade;

FIG. 4 shows a representation of a hub twisting configuration;

FIG. 5 shows the hub twisting configuration in more detail;

FIG. 6 shows a representation of an alternative hub twistingconfiguration;

FIG. 7 shows the alternative hub twisting configuration in more detail;

FIG. 8 shows a representation of a twisting mechanism located at bothends (hubs) of the blade;

FIG. 9 shows a representation of a twisting mechanism located at thecentre of the blade;

FIGS. 10 (a) to (f) show cross-sectional representations of proposedblade configurations;

FIG. 11 shows a view of a release type turbine perpendicular to the axisof turbine rotation;

FIG. 12 shows a view along the axis of turbine rotation; and

FIG. 13 (Detail A) shows a representation of a releasable end fixing fora blade.

The embodiments that will be discussed herein are intended to twistturbine blades out of optimum lift conditions, incorporating eitherstall or feathering conditions. The aim is to limit the rotational speedof the turbine, for example in high wind conditions. Twisting of theblades may occur naturally at a particular centripetal forcecorresponding to perhaps a maximum desired rotational speed.

Twisting to stall involves twisting in such a direction as to increasethe angle of attack sufficiently to induce aerodynamic stall. Twistingto feather involves twisting in the opposite direction, inducingfeathering by decreasing the angle of attack. Stalling may causeexcessive vibration of the blades to occur. Feathering does not producesuch vibration problems, however a much larger degree of twist isrequired.

As shown in FIG. 1 three aerofoil blades 2 are fixed at each end to hubs(4 and 5) mounted on a rotating shaft 3. The shaft will normally bemounted in bearings not shown and connected to a driven load such as anelectrical generator.

Each aerofoil blade 2 is made to be strong in tension but semi flexiblein bending.

In this example each blade is held firmly at one hub 4 end. The otherend of the blade is held in a rotating section 1. In this example therotation is induced by tension force in the blade due to centripetalforces on the blade as it rotates. The rotating section may be adjustedso that no rotation occurs until a threshold force is reached so thatthe blade stays in its preset (as shown) start up position until thispoint.

An example of one form of rotatable connector will be described, withreference to FIGS. 4 and 5. A short length of ball screw 6 is attachedto the blade end 7. The matching recirculating ball nut 8 is attached tothe hub. A torsion spring 9 is axially aligned along the ball screw 6axis and attached at one end to the hub 5 and at the other end to theblade end 7. The torsion spring 9 is wound against a rotation stop whichholds the blade end in the normal angular position and is wound upenough to prevent the ball screw 6 turning until the design rpm has beenreached for speed control to start. When this speed is exceeded the ballscrew 6 turns as the tension on the blade 2 creates enough force alongthe helical slope of the screw 6 to overcome the torsion spring 9preload. As the blade 2 is twisted by this action the net forwardaerodynamic forces on the blade 2 are reduced preventing furtherincrease in rpm. Preferably all three blade ends act in this way topreserve balance.

Another example of the rotatable connector is illustrated in FIGS. 6 and7. The blade 2 comprises 2 ropes (10 and 11), which run the length ofthe blade 2. One rope 10 is bolted to the hub 5 so as to provide a fixedpivot point. The other rope 11 is connected to a spring 12 or otherdamper such that when the threshold speed is exceeded, similarly to theabovementioned example, the spring tension is overcome and the blade 2is able to twist, with the bolted rope 10 acting as a pivot for saidtwisting.

The decision on which side is bolted and which side is connected to thespring will depend on whether a stalling or a feathering effect isdesired.

Another example of the rotational mechanism is a short spiralarrangement of spring lengths at the end of the blades such that eachindividual spring section is fixed at a helical angle to the hub at oneend and the blade at the other end, all forming a circularly displacedgroup. When tension is applied by the blade centripetal force theextension of the springs produces a rotational effect on the blade.

Another example of the rotational mechanism is an arrangement of twotriangular sections of stiff material with flexible links betweenarranged such that the axis of the links form a Z shape. Each of thethree link sections is folded in opposition to the adjacent such thatwhen one end link of the “Z” is attached to the blade and the other endlink of the “Z” is attached to the hub, when the blade is moved awayfrom the hub the folds open and the blade rotates with respect to thehub.

The spring retaining force can be by a separate attached spring or bymaking the links themselves of a spring material.

FIGS. 8 and 9 show different ways in which the rotatable connectortwisting mechanism may be deployed. FIG. 8 shows the rotatable connectorlocated at both hubs (4 and 5). FIG. 9 shows an alternativeconfiguration where the rotatable connector is located at the midpoint13 of the blade 2. Twisting at the midpoint 13 of the blade 2 may serveto reduce the extent of displacement required when compared to twistingat the hubs (4 and 5). Any of the twisting mechanisms herein discussedmay be suitable for locating at either hub, or indeed at the midpoint ofthe blades.

FIG. 10, (a) to (f), show various configurations of blade that may beadopted. (a) shows a blade consisting of a single rope 14 inserted in anaerofoil shaped cross-section rubber body 15. (b) comprises a doublerope 16 for added tensile strength. Such a blade may also be used withthe twisting mechanism of FIGS. 6 and 7. Multiple ropes or wires 17 mayalso be used for tensile strength and also to control the extent andconformity of the twist. Similarly, a double loop rope 18 might offerincreased tensile strength while still be suitable for the twistmechanism employed in FIGS. 4 and 5. 2 double loop ropes 19 offers ananalogous configuration for the embodiment of FIGS. 6 and 7. A yetfurther alternative embodiment utilises a hollow body 20 with a filler21. The hollow body is preferably of a fibre material to carry tensileloads, e.g. in the troposkien shape during operation.

The cross-section may be varied towards the hubs in order to smooth outthe variation in forward thrust depending on position along the axis ofrotation.

An embodiment of the present invention which incorporates means forreleasing one or more blades is illustrated in FIG. 11. This embodimentof the invention provides a fail safe mechanism and will preventrotation of the turbine in extremely high winds. This mechanism can beincorporated in a turbine containing means for twisting the blade inaccordance with the present invention. Three aerofoil blades 102 arefixed at each end to hubs (104 and 105) mounted on a rotating shaft(103). The shaft will normally be mounted in bearings (not shown) andconnected to a driven load such as an electrical generator.

Each aerofoil blade is made to be strong in tension but semi flexible inbending.

Each blade is held firmly at one end to hub 104. The other end of theblade is held in a releasing clamp 101. Blade release from the clamp isinduced by tension force in the blade due to centripetal forces on theblade as it rotates. This is calibrated to occur if other speed limitingsystems such as generator loading have failed and emergency overspeedprotection is needed.

To maintain rotational balance in the turbine all the clamps are linkedsuch that when one releases all the others are released.

One example of a releasing mechanism is to hold the blade ends in a slotwhich keeps them in the correct orientation. All the blades areprevented from pulling out of the slot by a loop of wire or cord ofknown breaking strength which is looped in turn through a hole or pin ineach blade. If the rotational speed of the turbine reaches overspeedcondition the loop breaks and all the blades are released from theslots.

Another example of a releasing mechanism is to hold all the releasableblade ends in a slot formed by the gap between two hub sections. Eachblade has a “detent” at its end that engages with a protrusion in onehub “half” to hold it in position. The force to keep the blades engagedis provided by a common spring or weight acting substantially along theaxis of the turbine shaft. The moving hub half is able to rock slightlyto apply equal force to all blade clamps. If the blade centripetaltension increases enough to pull the blade from one node of the clampthe resulting void allows the clamp to tilt and release the otherblades.

It is clearly advantageous to release all the blades simultaneously.

It is envisaged that a combination of the twist-type turbine and therelease-type turbine would provide a solution with inherent speedlimiting means and an emergency means for stopping the turbine if athreshold release speed was reached.

The present invention provides many advantages suitable for domesticimplementation of turbines. Turbulent airflows, such as are common indomestic environs, may be harnessed by vertical axis turbines.Additionally, the safety aspects of the invention, namely the velocitylimiting system and the emergency release that can be effected by therelease system, make the invention advantageous over HAWTs for domesticuse. There is also the significant advantage of reduced vibrationcompared to small scale HAWTs and previous Darrieus-type turbines.

Improvements and modifications may be incorporated herein withoutdeviating from the scope of the invention. For example, the inventionhas been exemplified by application to wind turbines. It is proposedthat the invention could be employed in other fluid mediums such aswater. Additionally, the twisting mechanism may be implemented by motorsor any other suitable control device.

1. A cross flow turbine comprising: one or more aerofoil bladesrotatably mounted about a central axis and connected to said axis at ornear each end of the one or more blades wherein said one or more bladesare provided with a degree of torsional flexibility such that they aretwistable about a longitudinal blade axis to reduce the aerodynamicefficiency of the one or more blades to control the rotational speed ofthe turbine.
 2. A turbine as claimed in claim 1 wherein, the one or moreblades are provided with a rotatable connector to allow the one or moreaerofoil blades to twist about the longitudinal blade axis.
 3. A turbineas claimed in claim 2 wherein, the rotatable connector couples the oneor more aerofoil blades to the central axis at one end of the one ormore blades.
 4. A turbine as claimed in claim 2 wherein, the rotatableconnector couples the one or more aerofoil blades to the central axis atboth ends of the one or more blades.
 5. A turbine as claimed in claim 2wherein, the rotatable connector is positioned a distance along thelongitudinal axis of the one or more aerofoil blades to couple twosections of the one or more aerofoil blades.
 6. A turbine as claimed inclaim 1 wherein, rotation of the one or more aerofoil blades is drivenby tension in the one or more blades caused by centripetal force.
 7. Aturbine as claimed in any of claim 2 wherein, rotation of the rotatableconnector is driven by tension in the one or more blades caused bycentripetal force.
 8. A turbine as claimed in any of claim 2 wherein therotatable connector is provided with a rotation inhibiting means thatprevents rotation below a predetermined centripetal force threshold. 9.A turbine as claimed in claim 8 wherein, the rotation inhibiting meanscomprises a torsion spring wound against a rotation stop which holds theone or more blades in place.
 10. A turbine as claimed in claim 8wherein, the rotation inhibiting means comprises one or more springsfixed at a helical angle to the central axis of rotation and to the oneor more blades at the other end.
 11. A turbine as claimed in claim 8wherein, the rotation inhibiting means comprises two triangular sectionsof stiff material with flexible links therebetween, said links forming aZ shape.
 12. A turbine as claimed in claim 1 wherein, the one or moreaerofoil blades are configured to twist in a predetermined directionwhen a tension threshold is reached.
 13. A turbine as claimed in claim 1wherein, the one or more aerofoil blades are configured to twist in afirst direction to feather turbine rotation.
 14. A turbine as claimed inclaim 1 wherein, the one or more aerofoil blades are configured to twistin a second direction to stall turbine rotation.
 15. A turbine asclaimed in claim 2 wherein, rotation of the rotatable connector isdriven by an actuator.
 16. A turbine as claimed in claim 15 wherein, theactuator operates at a predetermined threshold of central axisrotational velocity.
 17. A turbine as claimed in claim 15 wherein, theactuator is powered.
 18. A turbine as claimed in claim 15 wherein, theactuator is manually controllable.
 19. A turbine as claimed in claim 15wherein, the actuator is automatically controllable.
 20. A turbine asclaimed in claim 1 wherein, the torsional flexibility of the one or moreaerofoil blades are set at a predetermined level.
 21. A turbine asclaimed in claim 1 wherein, the one or more aerofoil blades are capableof adopting a troposkien shape during rotation about the central axis.22. A turbine as claimed in claim 1 wherein the one or more aerofoilblades comprise one or more flexible ropes enclosed by an aerofoilshaped profile.
 23. A turbine as claimed in claim 22 wherein, theaerofoil shaped profile contains a packing material to mechanically fixthe aerofoil shaped profile to the one or more ropes.
 24. A turbine asclaimed in claim 1 further comprising connection means provided at anend of the one or more blades which are releasably connectable to thecentral axis such that when speed of rotation of the turbine about thecentral axis increases to or over a predetermined threshold level theone or more blades are released.
 25. A turbine as claimed in claim 24wherein, the connection means is releasably connectable by means of aclamp.
 26. A cross flow turbine comprising: one or more aerofoil bladesrotatably mounted about a central axis and connected to the central axisat or near each end of the one or more blades by connection meanswherein the connection means provided at one end of the one or moreblades is releasably connectable and is released when speed of rotationof the turbine about the central axis increases to or over apredetermined threshold level.
 27. A turbine as claimed in claim 26wherein releasing one end of the one or more aerofoil blades causes theforward driving force of the one or more blades to be reduced.
 28. Aturbine as claimed in claim 26 wherein excess tension in the one or moreblades due to centripetal forces caused by excess rotational speedcauses the one or more blade ends to be released.
 29. A turbine asclaimed in claim 26 wherein the one or more aerofoil blades areflexible.
 30. A turbine as claimed in claim 26 wherein the connectionmeans is releasably connectable by means of a clamp.
 31. A turbine asclaimed in claim 26 wherein each of the one or more aerofoil blades arereleasably connectable and wherein release of the one or more bladesoccurs upon reaching said predetermined speed of rotation threshold. 32.A turbine as claimed in claim 26 wherein the one or more blades arereleased substantially simultaneously.
 33. A turbine as claimed in claim26 wherein a single mechanism is used to release all of the one or moreblades.
 34. A turbine as claimed in claim 26 wherein when the one ormore blades are released they swing out under centripetal forces.
 35. Aturbine as claimed in claim 34 wherein release of the one or more bladesimmediately slows the turbine.
 36. A turbine as claimed in claim 35wherein further slowing then occurs due to the adverse aerodynamicgeometry of the one or more blades when held at one end only.